feat(server): go.mod + config + modèles de données

Initialise le module Go github.com/user/nanometrics/server avec toutes
les dépendances (SQLite, gorilla/websocket, paho.mqtt, prometheus,
imaging). Ajoute config.go (Load/Default via env vars) et models.go
(AgentMetrics, SmartMetrics, Agent, AgentConfig, ServerConfig, WSMessage).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/config/config.go

@@ -0,0 +1,38 @@
+package config
+
+import "os"
+
+type Config struct {
+	UDPAddr       string
+	DBPath        string
+	HTTPAddr      string
+	MQTTBroker    string
+	MQTTTopicBase string
+}
+
+func Load() Config {
+	return Config{
+		UDPAddr:       getEnv("UDP_ADDR", "0.0.0.0:9999"),
+		DBPath:        getEnv("DB_PATH", "/data/nanometrics.db"),
+		HTTPAddr:      getEnv("HTTP_ADDR", "0.0.0.0:8080"),
+		MQTTBroker:    getEnv("MQTT_BROKER", "tcp://10.0.0.3:1883"),
+		MQTTTopicBase: getEnv("MQTT_TOPIC_BASE", "nanometrics/agents"),
+	}
+}
+
+func getEnv(key, fallback string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return fallback
+}
+
+func Default() Config {
+	return Config{
+		UDPAddr:       "127.0.0.1:19999",
+		DBPath:        ":memory:",
+		HTTPAddr:      "127.0.0.1:18080",
+		MQTTBroker:    "tcp://127.0.0.1:11883",
+		MQTTTopicBase: "test/nanometrics",
+	}
+}